Project Zeta Sigma (unproduced TV story)
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Project Zeta Sigma was an unproduced serial that would have been the first story of the Fifth Doctor. It would not have carried on straight from Logopolis and would have featured nuclear disarmament, an alien cold-war scenario with The Master orchestrating it. John Flanagan and Andrew McCulloch co-wrote the serial, which was stopped 1981 by Antony Root as new Script editor with the approval of Barry Letts in the absence of John Nathan-Turner.
In another version, John Nathan-Turner claimed that it would not have worked and Castrovalva replaced it. (REF: Doctor Who - The Handbook: The Fifth Doctor)
